Permeability tuned intermediate frequency transformer



3mm WILLIAM R. FAUST W. R. FAUST ET AI. PIRIBABILITY TUNBD IIITBRHBDIATB PRBQUBHGY TRANSMEM FilOdLpril 28, 1945 HowARp o. LoRENzEN 5.,' (bag,

l Patented Aug. 17, 1948 PERMEABILITY TUNED INTERMEDIATE FREQUENCY TRANSFORMER William R. Faust and Howard 0. Lorenzen, Washington, D. C.

Application April 28, 1945, Serial No. 590,869

5 Claims.

(Cl. Pil- 119) (Granted under the act of March 3, 1883, as amended April-30, 1928; 370 0.` G. 757) The present invention relates to tunable interstnge transformers and more particularly to inimmediate-frequency permeability-tuned transformers.

According to the known practice, intermediatefrequency transformers in superheterodyne receivers are adjusted to give the desired L-C combination for the plate and grid .circuits of the driving and driven tubes respectively, by varying the position of a coil core element of high permeability, such as an iron-dust core, in the primary and secondary windings. Such adjustments become necessary whenever any of the constants of the circuits are altered as upon insertion of a new tube, a condenser or resistor.

However, in such practice, it has been found that, when the iron cores are moved relative to their respective transformer windings, the coupling coefiicient of the transformer is altered,

with consequent alteration of the over-all, amplio fication-characteristic of the amplifier in which the transformer coupling is used.

An object of the present invention is to provide l a coupling transformer overcoming the above disadvantages of the known practice.

Another object is to provide a coupling transformer in which the inductance of either or both the primary and secondary circuits may be varied without undesirable variation in the coupling cocicient of the transformer.

A further object of the invention is the provision of a coupling transformer having coupling and tuning windings with minimum inductive coupling between a tuning winding and the coupling winding.

Other objects and advantages of the invention will become apparent upon a perusal of the following specification and the drawings accompanying th'e same.

In the drawings:

Fig. l is a schematic diagram of the arrangement of windings and core elements constituting the transformer.

, Fig. 2 is a side elevation of a preferred embodiment of the invention.

Fig. 3 is a section on the line 3-3 of Fig. 2 with certain central portions in full.

Referring to the drawings more in detail, and first to Fig. l, here is illustrated the principle of the invention which comprises in general a primary section' I0 and secondary section II, the

coupling portion I4 and a. tuning portion I5. 5

2 The coupling portions I2 and I4 are relatively closely coupled while the tuning portions I3 and I5 are relatively loosely coupled and preferably very loosely coupled. To further. avoid objectionable -coupling between the. tuning portion of one section of the transformer and the coupling portion of the other, say the portions I3 and I4, and to afford close coupling between the portions I2 and I4, and to give them a relatively large inductance without undue axial length, the coupling portions I4 and I2 are given a relatively large diameter. However, it is not essential that the portions I2 and I4 be of larger diameter than the tuning portions, and in fact may be of evenv slightly less diameter than the tuning coils without detriment to the operation of the device.

Each of the tuning portions I3 and I5 is provided with' means I6 and I'I respectively for varying its inductance, the inductance varying means being preferably in the form of iron-dust cores movable toward and Iaway from the centers of their respective tuning-coil portions. Thus movement of the core `II toward and away from the center of the coil portion I5 will increase and decrease the eiective inductance of the latter with consequent variation in the tuning of any circuit of which th'e latter forms a part, and this without substantial variation of the coefficient of coupling, due to the remoteness of the coil portion I5 from the primary coil-portions I2 and I3. Mutual inductance between coil-portions I4 and I5 is maintained low and practically ineiectual by suitable spacing between them and by avoidance of a movement of the core I1 too far into the space between the coil portions. Preferably, movement of the core into the tuningcoil portion beyond the center of th'e latter is to be avoided and to insure against further movement, suitable mechanical stop means I8 of substantially non-magnetic, non-conducting material may be provided.

Referring now to Figs. 2 and 3 which show one practical, physical embodiment of the invention, the main frame consists of a tube member I9 of electrical insulating material, such as polystyrene, mounted in a pair of end plates 20 and 2l of like material and having mounted on its central portion an enlarged coil support or form 22, in the form of a short section of tubing of electrical insulating material telescoped over the inner tube I9 and suitably secured th'ereto as by cement or other suitable securing means. This relatively large central core-form supports the two, closely coupled, coupling portions I2 and I4, while the inner tube I9 supports the tuning portions I3 and I5, of the primary and secondary sections of the transformer. In the present embodiment, the large coils have a diameter oi' approximately one and one half times that of the small coils, the actual diameter being approximately one half of an inch for the outside diameter of the small tube I9 and three quarters of an inch for the outside diameter of the large tube.

Aspointed out above, this difference in diameter is not essential to satisfactory operation of the device, and the device may as well be formed with the coupling coils-wound directly on the tube member I9'so long as th'e coil section I3 is Ispaced sufficiently distant from the coil section I4 to have only a relatively loose coupling therewith and the coil sections I2 and I5 are simimutual inductance betwe in I'| and I3 because of their remoteness from each other, or in the mutual inductance between I1 and I2 because of their remoteness from each other. With various of these adjustments within wide limits, the coeilicient of coupling between primary and secondary sections of the transformer will remain substantially constant, due to the fact that although movement of either inductance-varying core toward the center of the transformer structure iny creases very slightly the coupling .between the larly spaced. The coupling coils I3 and I4 are formed with closely spaced turns, for example, contacting convolutions of 'enameled wire, while the small coils are formed with more widely spaced convolutions, approximately the diameter of the wire apart. This relatively large spacing of the tunable coil portion results in a more gradual variation of inductancel for a given extent of movement of th'e iron-dust core element.

Terminal lugs such as 23, 24 secured in the lower end plate 2 I, serve as connecting terminals for the coils. As shown in Fig. 2, the lugs 23 and 24 serve as terminals for the primary section of the transformer through leads 25 and 26 while a similar pair of lugs, not shown in Fig. 2, situated in line with the first mentioned lugs as indicated for the lugs 23, 2l in Fig. 3, serve as terminals for the secondary portion through leads 28 and 29 respectively.

The inductance varying means for the tuning coil-portions. I3 and I5 are here embodied in the form of iron-dust core elements I6 and I1 mounted for axial adjustment Within the inner tube I9 to and away from the centers of their respective tunable coil elements by means of threaded supporting rods 30 and 3| respectively, each rod passing through an end cap such as the cap 32, in threaded engagement with the cap and terminatim; in a slotted end portion 33 outside the cap to facilitate adjustment with a screw driver. Each end cap such as the end cap of 32 is held ilxed in the end of the inner tube in any suitable manner as by a driving fit of its rough-surfaced reduced portion 34 within the tube end. A spring clip 35 engages the rod 3| through a slot in the end cap to friotionally hold the rod in adjusted position. The stop element I8 is comprised of a cylindrical plug of electrical insulating material cemented or otherwise secured in the inner tube I9 with its ends extending to approximately the middle of the tuning coil-portions I3 and I5.

In operation, assuming for example that the transformer is used as a coupling transformer in an I.-F. amplifier circuit, when some element of the circuit is altered as when a new tube is insertcd. say in the circuit of the secondary section Il of the transformer, to replace a poor one, the capacity of the new tube may not be the same as that Vnl? the original. To maintain the circuit tuned to the desired intermediate frequency, it will he necessary to increase or decrease the inductance of the secondary circuit, according as to whether the capacity of the new 4tube is less or greater than that of the original tube. This may be accomplished by moving the plug I1 toward or away from the center of the tuning coil-portion I5. It will be clear that this adjustment can take place with but an extremely slight change in the coil-portion thus adjusted and the opposite side of the transformer, tending slightly to increase the mutual inductance between the two sides of the transformer as a whole, and consequently the coe'illcient of coupling, the increase in the inductance of the adjusted coil-portion tends to reduce the coefficient of coupling as between the two sides of the transformer. This will be further clarified upon reference to the formula for the coefiicient of coupling K, where ...L \/L10X Ln in which M is the mutual inductance between the primary and secondary sections I0 and II, and L10 and L11 the self inductance of the sections I0 and II, respectively.

In the ordinary transformer in which one or both of the windings consisting of a single primary and a single secondary arranged end-toend on a common tubular support, are permeability tuned by insertion of a core, the mutual inductance tends to increase when the magnetic cores are moved into the coils. Then the lower term of the equation also increases but their rates of change are different, consequently K will vary,

In the new transformer the eiective mutual inductance M between the primary and secondary sides of the transformertaken as a whole consists of a constant part, that between the coupling portions I2 and I4 plus a variable part, that between I3-I5, I3-I4 and I2-I5, small relative to the constant part. This variable part of the total mutual inductance M increases when the cores are moved in. Obviously the rate of change of M has been changed from what it was in the old arrangement, and by proper adjustment of the constant part of the mutual inductance relative to the variable part, the rate of increase in M may be made so small relative to that. of Lio and L11 thaiI it will just make up for the increase in the lower term of the right hand side of the equation andformer as a whole, the effective M will be constant and the term, square root of LwXLn, will also be relatively constant because the changes in Licanfi L11 are assumed small. Therefore the coefficient of coupling will be constant as is also the mutual inductance.

It is to be noted here that in use, the effect of the distributed lcapacity between the couplingr coils I2 and I4 may be rendered either aiding orV opposing with reference to the coupling effect between these coils, in known manner as by reversal of the connection -to the leads of one of the coils. and according as to what shape selectivity curve or frequency-response curve is desired.

While one specific embodiment of the invention has been described herein in detail for the sake of disclosure, it is to be understood that the invent-ion is not limited to such specific embodiment but contemplates all such modifications and variants thereof as f-al1 fairly within the scope and purview of the appended claims.

'I'he invention described herein may be manufactured and used by or for the Government of the Uni-ted States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed Iis:

1. A tuned transformer comprising a pair of relatively closely coupled primary and secondary coil-portions, a third coil-portion having a looser coupling to said primary and secondary coil portions than the coupling between said prlmary and secondary coil-portions and connected in series with the nearer of said primary and secondary coilV portions, and inductance varying means for said third coil portion.

2. A Ituned transformer comprising a pair of relatively closely coupled primary and secondary coil-portions, a third coil-portion having a looser coupling to said primary and secondary coil portions than the coupling between said primary and secondary coil-portions arid connectedA in series with the nearer of said primary and secondary coil-portions, and inductance varying means for said third coil-portion, all said coil-portions being spaced axially along a common axis and said closely coupled portions being of larger diameter than said third coll-portion.

3. A tuned transformer comprising a pair of relatively closely coupled primary and secondary coil-portions, a third coil-portion having a looser coupling to said primary and secondary coil portions than the coupling between said primary and secondary coil-portions and connected in series with the nearer of said primary and secondary coil-portions, all said coil-portions being spaced axially along a common axis and said closely coupled portions being o! larger diameter th-an said third coil-portion, inductance varying means for said third coil-portion movable along the common axis into dilerent degrees of operative relation with said third coil-por-tion, and means for limiting the move-ment of said inductance varying means to a degree of movement insuilicient to bring it into substantial cooperative relation with said nearer coil-portion.

4. A permeability tuned transformer comprising a tubular coil-form of electrical insulating material, a group of four coil sections spaced axially along .the outside of said form, the two middle sections of said group being relatively closely coupled and the end sections each relatively loosely coupled with the nearer of the middle coil sect-ions and connected in series with only the nearer of said middle sections and a pair of ferromagnetic high permeability core elements slidably mounted in the tubular coil-form, one from each end, toward and away from the center of a nearend coil-section.

5. A permeability tuned transformer comprising a coil-forrn of electrical insulating material, a'group of four coil sections spaced axially along said form, a pair of inductance varying means one for each'end coil of the group, each end coil being relatively closely coupled to its inductance varying means and relatively very loosely coupled to both middle coils of the group and the end coil and sa-id middle coils being relatively closely coupled to each other, whereby the induction varying means has relatively very slight but substalntially equal influence on both said middle co s.

WILLIAM R. FAUST. HOWARD O. LORENZEN.

REFERENCES CITED The following references are of record in the ille of this patent:

UNITED STATES PATENTS 

